1. Field of the Invention
The present invention relates to a semiconductor structure and method of making the same. More specifically, this invention is related to quantum well structures, including resonant tunneling devices fabricated on a semiconductor substrate.
2. Description of the Related Art
Modern day electronic systems are capable of operating at very high speed based on fast-switching integrated circuits. However, there are always some circuits associated with these electronic systems that must operate at an even higher pace. A telecommunication system is such an example. In such a system, electrical signals are transmitted or received serially through a transmission medium via an antenna, or an optical or electrical cable. However, the serially received signals are processed parallelly in the main system. Thus these front-end circuits directly linked to the transmission medium must operate at a speed higher than the main system in order to avoid transmission bottleneck. In one example of a digital system, the main system processes thirty-two bits of digital data plus a few bits as parity checks in one clock cycle. These front-end circuits must operate at least thirty-two times faster than the main circuits for an optimal overall system performance. As these main processing systems are more and more designed towards the parallel architecture, there will be an increasing demand for faster electronic components for the implementation of such circuits.
A transistor's speed performance can be gauged by a parameter called Cut-Off Frequency (f.sub.T). Cut-off frequency is defined as the frequency for which the output to input gain ratio of the device is maintained above 3 db. Silicon-based circuits can be used for the front-end circuits mentioned above. However, mobilities of charge carrier in silicon are inherently slower than other semiconductor materials. One specific area that shows prominence is the development of Metal Semiconductor Field Effect Transistors (MESFET) utilizing the group III-V compounds. A conventional depletion mode GaAs (Gallium Asenide) MESFET can easily surpass a cut-off frequency of 10 GHz.
To further enhance performance, several variations of a MESFET were evolved in the past, such as the High Electron Mobility Transistor (HEMT). A HEMT is basically the same as a conventional MESFET, except a heavily doped layer is superimposed on top of an undoped layer in the semiconductor substrate. The bandgap of the doped layer is larger than that of the undoped layer. When the two materials are structurally brought together, their Fermi levels are aligned with each other. As a result, the conduction band is at a higher energy level in the doped material in comparison with its counterpart in the undoped material. During normal operation in the conduction mode, electrons from the source of the HEMT migrate into the heavily doped layer but are attracted into the undoped layer due to the inherent tendency of electrons to travel to a region with a lower energy state. The electrons travel in the form of a thinly charged sheet called the Two Dimensional Electron Gas (2DEG). The 2DEG is substantially parallel to and in close proximity with the boundary surface of the doped and undoped layer, but is located in the undoped layer. The undoped layer, being relatively free of charge scattering interferences as compared to the doped layer, enables the electron to travel with a higher mobility. Thus HEMTs are capable of operating at a higher speed than conventional MESFETs.
Another approach to improve the operational speed of MESFETs or HEMTs is to build them with smaller geometries, to reduce the parasitic capacitances. The source and drain are disposed closer together on the semiconductor substrate, with the channel assuming a much shorter length. Consequently, the gate contact traversing across the channel length is also scaled down in cross-sectional area. As a result, the gate resistance is increased, which in turn degrades the device's performance. To reduce the gate resistance, a goblet-shaped gate contact with a larger cross-sectional area called a T-gate is disposed atop the channel. In conjunction with the smaller dimensions, this produces a faster device with lower noise. Self-alignment of the source and the drain with respective to the T-gate further helps to increase the device's speed limit.
In an attempt to further increase speed and possibly to break through the tera Hertz barrier, Resonant Tunneling Transistors (RTTs) designed as a HEMT have been proposed. A RTT requires only minimal voltage swings as the signal control on the gate terminal. There is no need to sweep the gate with high voltage amplitudes to direct a RTT in and out of resonance. This feature enables the resonant tunneling structure to function as a high speed switching device.
To understand the operation of a resonant tunneling structure, the behavior of electrons must be briefly explained. Each electron in the conduction band assumes its own discrete energy state in an individual atom. However, when the atoms are brought together, wave functions of the electrons begin to interact with each other. From the Principle of Exclusion, no two electrons can occupy the same energy state in a closed energy system. As a consequence, multiple energy states exist in the form of a quasi-continuous spectrum in the conduction band. However, when fewer atoms are confined in a restricted space, the number of energy states is reduced and the energy states become more distinctly separated from each other. The restricted space is called a quantum well, wire, or dot depending on the number of the confinement of the electron mobility. The separated energy states are called sub-bands within the quantum well. The smaller the physical size of the quantum well, the more distinct are the separations of the sub-bands. This phenomenon is called the confined quantum effect.
In practice, a quantum well is enclosed within potential barriers in a semiconductor material. In this specification and in the appended claims, the term "potential barrier" is defined as an isolation between two closed energy systems, with each closed energy system asserting its own potential distribution for its charge carriers. The term potential barrier includes resistive barriers, field-induced barriers, or simply physical gaps devoid of materials. In a quantum diode, for example, a thin layer of semiconductor material is sandwiched between two resistive layers which act as potential barriers. FIG. 1 illustrates such a resonant tunneling diode which is signified by reference numeral 2. Quantum well 4 is sandwiched between first and second potential barrier layers 6 and 8. During resonance, electrons from cathode 10 traverse first barrier 6 into quantum well 4, and pass through second barrier 8 and reach anode 12. First and second metal contacts 14 and 16 allow resonant tunnelling diode 2 to electrically communicate with other devices. Due to the difficulty with forming an electrical contact with quantum well 4, the quantum well potential can not be controlled. As a consequence, a two-terminal device can only be built which can not be used as a digital switch or as an amplifier.
To alleviate this drawback, three terminal resonant devices were designed in the past. An example of such a device is shown in FIG. 2. Resonant tunneling transistor signified by reference numeral 18 is formed on a heterojunction structure 19. Quantum well 34 is defined within first and second voltage-induced potential barriers 28 and 30 controlled by first and second barrier control terminals 24 and 26, respectively. Potential barriers 28 and 30 are basically inverted semiconductor regions induced by the bias voltage on control terminals 24 and 26. It should be noted that potential barriers 28 and 30 disappear once the bias voltage is removed. Control terminals 24 and 26 are formed on the semiconductor substrate 19 via the conventional method of e-beam lithography and lift-off. The detailed structure and method of fabrication of this type of resonant tunneling device can be found in J. N. Randall et al., "Fabrication of Lateral Resonant Tunneling Devices", Journal of Vacuum Science Technology, B 10(6), Nov/Dec 1992, pages 2941-2944. A device of similar construction is also disclosed in S. Y. Chou et al., "Lateral Resonant Tunneling Filed-Effect Transistor", Applied Physics Letters, 52(23), Jun. 6, 1988, pages 1982-1984. In FIG. 2, the potentials of the sub-bands in quantum well 34 is controlled by control gate 22. When the sub-bands are in alignment with the Fermi level of the source, electrons tunnel through the potential barrier 28, quantum well 34, potential barrier 30, respectively, and the device is said to be in resonance. Thus RRT can be used as a switching device. However, the RRT thus fabricated, is relatively large in size. The reasons are twofold. First, control terminals 24 and 26, normally defined by the conventional lithographic process, are limited in resolution. A typical dimension for the terminals 24 and 26 fabricated with e-beam lithography is of the order of 30-60 nanometers. Most detrimental of all is side-fringing effect of the electric field underneath control terminals 24 and 26, which tends to spread out the field-induced potential barriers 28 and 30 laterally. Excessive spreading of the induced electric field may cause barriers 28 and 30 to bridge with each other, resulting in the elimination of the quantum well 34. To avoid the merging of the barriers, the control terminals 24 and 26 must be widely separated. As was previously mentioned, large quantum wells do not have sharp and well defined sub-bands. Consequently, the sub-bands within the relatively large quantum well 34 are not very distinctly defined. Degradation of the device performance follows with the most pronounced effect being the reduction in peak-to-valley current ratio and considerable thermo-excited leakage current during the out-of-resonant state. These drawbacks seriously degrade the device performance which renders the device to be practical only in the low temperature range. Typically, functional performance with current densities below 110 A/cm.sup.2 can only be obtained under 20 degrees Kelvin with this type of structure. As a comparison, an RTT may need to deliver a current density in excess of 10.sup.4 A/cm.sup.2 to be considered as a practical device. Moreover, there is considerable gate resistance embedded within gate contact 22 as its thickness is typically less than 100 nanometers. In addition, source 21 and drain 20 are not fabricated with a self-aligned process with respect to the gate 22. Consequently, source 21 and drain 20 are spaced away from each other resulting in higher source resistance. The parasitic capacitances resulting from the inherently large-sized device also limit its frequency response. Devices of this type, although exhibiting some confined quantum effects at low temperature, are difficult to be used in practical applications due to its slow speed, high noise output, and the low temperature operation requirement.
Experiments have been performed in the past to study the confined quantum effect of electrons in a quantum well defined by ion-implanted barriers. One such research work is reported in Nakata et al., "Fabrication of Quantum wires by Ga Focused-Ion-Beam Implantation and Their Transport Properties", Japanese Journal of Applied Physics, Volume 29, No. 1, January 1990 pages 48-52. In Nakata et al., the magnetoconductance of electrons in a one-dimensional quantum wire is studied. Micro-probes are used to launch, receive, and detect the charge carriers. Having only one quantum well yields very limited current density. Furthermore, the associated probes substantially introduce parasitic inductances and capacitances to the device, which render it difficult to be operated under high speed operation. The structure in Nakata et al. can not be used as a practical device.